Glass fibers and fiber-optics devices may be used to route, distribute, and deliver laser beams from single or multiple laser sources to different destinations, including devices, machines, and systems that are located a distance from the laser source. The fiber-based devices are also widely utilized for intentional manipulation of laser beam characteristics inside the fiber (e.g. laser light power, polarization, phase). Laser beams propagate in fibers and fiber-optics systems over certain distances prior to being transmitted into air or other media through a tip of the fiber end-section, which may be referred to herein as the delivery fiber. The optical fiber itself may have a core and cladding areas having slightly different refractive index based on the material of their construction. Both the fiber core and cladding may provide guiding of laser light components known as modes, or guided transversal modes. The fibers that preserve propagation of a single-mode (SM) laser beam may be referred to herein as SM fibers. The SM fibers can be made to also maintain, substantially unchanged, the polarization state of the propagating in fiber light. These fibers are referred to as polarization maintaining (PM) SM fibers. In the so-called multi-mode fibers, the fiber core is larger compared to the SM fibers ranging from tens to hundreds of microns. These fibers may be known as large mode area (LMA) fibers and the multimode fibers guide either a few (few-mode fibers) or a number of transversal modes that comprise laser light inside the fiber and immediately after exiting the delivery fiber tip. The multimode fibers allow for much easier launching of light inside the fiber and can provide guiding of laser beams with significantly higher power (up to and above 10 kilowatts) if compared with the SM fibers. At the same time SM and LMA fibers provide better quality laser beam (e.g., a single-mode Gaussian shape beam) that has smaller divergence after exiting the fiber delivery fiber tip, which may be advantageous and highly desired for many applications.
Factors such as imperfections in fiber manufacturing, different operations with fibers (e.g., splicing of different fibers, fiber modification for stripping of high-order modes to maintain SM operation of a fiber system, tapering of fibers), environmental effects such as vibrations and temperature fluctuations, mechanical factors (e.g., fiber twisting and or bending), acoustical disturbances, and other factors may cause deviations in characteristics of the laser light that is transmitted from the delivery fiber tip. Examples of deviations from the expected or desired laser beam characteristics may include, for example: (a) temporal fluctuations of the transmitted laser power, polarization state, piston phase that is associated with deviations in optical path length in fiber systems; (b) appearance of undesired laser irradiance components such as high-order modes, light components coming through the fiber cladding (e.g., cladding light), laser light used as a pump in fiber amplifiers and fiber lasers (e.g., residual pump light); (c) changes in the transmitted light characteristics that are associated with non-linear effects in fibers such as the stimulated Brillion scattering (SBS) and non-linear phase modulation; (d) changes in the transmitted light that are related with external light that is received through the fiber tip (e.g. back reflected light).
It may be advantageous for such deviations from the expected or desired transmitted light characteristics to be detected in situ.